Abstract Primary (thymus) and secondary (spleen) murine lymphoid tissues express a 25-kDa protein that binds IGF-I. To determine the cellular source of this insulin-like growth factor binding protein (IGFBP), 11 murine or human cell lines representing T, B, and myeloid cells at various stages of differentiation were characterized by IGF-I affinity cross-linkage and Western ligand blotting. Mature myeloid cells, but not T or B cells, secrete a 25-kDa protein that is capable of binding IGF-I. CSF-1-derived bone marrow macrophages also synthesize this 25-kDa IGFBP. Thymic macrophages, which were estimated to secrete 2 ng of binding protein/10(6) cells-h, were used in conjunction with [125I] IGF-I affinity cross-linking to develop a protein binding immunomobility-shift assay to identify which IGFBP is produced by these cells. An anti-IGFBP-4Ab, but not an anti-IGFBP-2 Ab or normal rabbit serum, shifted the [125I] IGF-IGFBP complex to a higher m.w. position, indicating that the single 25-kDa IGFBP is IGFBP-4. Northern blotting confirmed that transcripts for IGFBP-4 as well as IGF-I are expressed in thymic macrophages. A putative 278-bp IGFBP-4 cDNA fragment (residues 341-618) of rat) that contains two unique cysteine residues found only in IGFBP-4 was cloned and sequenced from thymic macrophages. These clones differed from the rat sequence at only six residues (97% homology), and the deduced amino acid sequence from the murine cDNA was identical with that of the rat sequence. Subsequent studies revealed that IGF-I stimulates DNA synthesis in thymic macrophages. However, two different IGF-I analogues differing in the amino-terminus that bind equally well to the IGF-I receptor but poorly to IGFBPs are as effective as IGF-I at 100-fold lower concentrations. These data demonstrate that murine macrophages are a source of a single 25-kDa secreted protein that binds IGF-, that the molecular identity of this protein is IGFBP-4, and that this binding protein may antagonize the extracellular effects of IGF-I.